This species is extremely hard to sex. Female epalzeorhynchus bicolor sharks are usually larger than the males. The female will have a full, wide stomach and the males will be more slender.
Out curiosity why do you want to know the sex of this fish? If it's because you want to breed them, the red-tailed shark is extremely hard to breed in the home tropical fish aquarium. Most specimens that purchased in our local fish stores are bred in commerical fish farms by artificial means in Asia.